The present invention relates to apparatus for cooling and/or remoistening of a moving web. In drying a moving web of material, such as paper, film or other sheet material, it is often desirable that the web be contactlessly supported during the drying operation in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example.
It is often necessary to cool and/or remoisten the web after it has been dried. For example, U.S. Pat. No. 5,333,395 discloses a drying apparatus for traveling webs which includes a cooling tunnel directly connected with the dryer, a combustion chamber for combusting solvent which becomes volatile during drying of the web, heat exchangers, etc. U.S. Pat. No. 5,038,495 discloses a cooling device for cooling a web of material exiting a dryer. The cooling device comprises a substantially closed housing with an inlet and an outlet slit for the web of material. The housing includes a feed aperture at the outlet slit side for feeding outside air into the housing, and a discharge aperture at the inlet slit side for discharging air from the housing into the dryer. Air is fed through the housing counterflow to the direction of web travel. A series of nozzles bring the infed air into contact with the web of material. U.S. Pat. Nos. 4,702,015, 4,689,895 and 4,763,424 disclose apparatus and a method for providing a shower of fog onto a web or machine component in contact with the web. The fog evaporates on the hotter surface and cools that surface. U.S. Pat. No. 5,881,647 discloses the use of electrostatic fields to direct a water spray towards a web.
Smoke tunnels are conveniently used in web dryers to address the generation of smoke during processing. Smoke tunnels are typically located between the flotation dryer and the chill stand. More efficient handling of the web as it exits the dryer would be desirable.
It therefore would be desirable to lower the bulk temperature of the web in order to decrease the heat load of the cooling or chill rolls, or even eliminate the chill rolls or other cooling means such as an air-based cooling zone. Lowered web bulk temperature also would decrease the evaporation rate of the solvent mixture coating the web, thereby reducing the visible vapors evolving from the web and eliminating the need for a smoke tunnel. Condensation that normally occurs at the dryer exit and on the cooling rolls could be controlled to a minimum, and the product quality of the web could be improved in view of the absence of excessive moisture loss from the web.
The problems of the prior art have been overcome by the present invention, which provides apparatus and a method for enhancing the effectiveness of a water spray to cool and/or remoisten a web of material. More specifically, the preferred embodiment of the apparatus in accordance with the present invention includes a web dryer, preferably a flotation dryer, an electrostatic charge generating device or devices, a water spray, an integrated power supply to supply high voltage power to the charge device, and a drainage system for handling excess liquid generated during the cooling and/or remoistening of the web. The cooling apparatus is capable of a modular arrangement to optimize spacing and facilitate the addition of cooling capacity where needed, such as with faster web speeds or heavier web weights. One or more temperature sensors can be used to optimize the amount of and rate of fluid fed to the spray nozzles. The spray nozzles and charge bars can be retractable with respect to the web to facilitate web up procedures.
In its method aspects, the present invention is directed towards a method for drying and cooling a web, typically a moving web, by heating the web such as with a plurality of air flotation nozzles, and enhancing the effectiveness of a water spray by directing the water spray onto the web, by controlling the amount of water sprayed onto the web based upon the web temperature, by removing excess liquid generated during the cooling and/or remoistening process, and by removing steam generated by the cooling process.
The present invention thus reduces or eliminates dryer smoking from the dryer exit end without the use of a conventional smoke tunnel, reduces or eliminates solvent condensation problems on the chill rolls, reduces chilled water use at the chill stand, has less sensitivity to dryer/printing press process variations, and has lower dryer operating temperature and associated web exit temperature. It is also believed that with paper webs, the paper gloss is improved and the handling in the folder is improved due to slightly higher residual solvent in the paper web.
Successful operation of the unit requires careful management of the cooling fluid spray. It is important to monitor the web temperature and use that temperature measurement to limit the amount of spray. Excess spray is not desired as it can migrate into the upper part of the hood and reduce the life of the charge bars. In addition, extra mist can pressurize the unit and leak out the web slot and condense on chill rolls and other surfaces. This mist may also contain solvent vapor which forms deposits on the surfaces it condenses on.